1. Field of the Invention
The field of the invention is that of very-long-distance digital transmission (several thousands of kilometers) on optical fibers using on-line optical amplification. The invention can be applied, for example, to the setting up of trans-ocean links.
One of the main factors limiting the bit rate in very-long-distance systems such as these is the distortion induced by the transmission fiber.
The invention relates notably to compensation for this distortion. More specifically, the system according to the invention relates to compensation, by anticipation at the source of transmission (i.e. at the transmitter station), for all the disturbances induced by the fiber and notably for the non-linear effects.
Indeed, the distortion given by the transmission fiber to the transmitted signals arises, in monomode fibers, out of the combined existence of two phenomena: chromatic dispersion and non-linear effects.
The first phenomenon is that of chromatic dispersion. This phenomenon results from the frequency dependency of the refractive index of silica. It entails different propagation times depending on the operating wavelength. In general, chromatic dispersion tends to widen the pulses of the digital trains and, hence, to prompt inter-symbol interferences.
In the usual fibers, the chromatic dispersion is zero around 1.3 .mu.m and takes a positive value of about 17 ps/nm/km around 1.55 .mu.m. It is also possible to use dispersion-shifted fibers which are designed to have zero chromatic dispersion in the region of 1.55 .mu.m.
It has to be noted that the effect of distorsion by chromatic dispersion depends greatly on the spectral components of the pulses. Thus, if a pulse shows phase variations that are positive at its start and negative at its end, then this pulse will be greatly widened by a positive chromatic dispersion (the converse is true for negative dispersions).
Very-long-distance transmission systems (covering several thousands of kilometers) work at 1.55 .mu.m. The excessive value of the chromatic dispersion of the usual fibers at this wavelength rules out their use. Hence, dispersion-shifted fibers are used systematically.
The second phenomenon relates to the non-linear effects. The most important non-linear effect in a fiber is the Kerr effect. This effect, which is described for example in the document by K. W. Blow and N. J. Doran, "Non-linear Effects in Optical Fibers and Fiber Devices" (IEEE Proceedings, June 1987, pp. 138-144) reflects a linear dependency of the refractive index of silica with respect to optical power.
This is very low in the usual fields of operation of the optical systems (distance smaller than 400 km and power below about 10 mW), but becomes non-negligible for very high power values (of the order of 1 W) or for very large propagation distances at reasonable levels of power (some thousands of kilometers in a periodic amplification system).
The non-linear effects give rise to a self-phase modulation. When a pulse is transmitted through a fiber with positive dispersion, it gets widened and the high frequency components are pushed towards the front or leading edge of the pulse. In a transmission through a fiber with negative dispersion, the width of the pulse is equally well increased but, in this case, the high-frequency components are pushed towards the rear or trailing edge of the pulse.
The distortion provided by the transmission fiber should be considered as the combination of the chromatic dispersion (the first phenomenon) and of the non-linear effects (the second phenomenon).
The combination of these two effects may be described by a non-linear equation with partial derivatives of distance and time, known as Schrodinger's nonlinear equation, the resolving of which is discussed notably in the work by G.
Agrawal, "Non-Linear Fiber Optics", Academic Press.
The numerical resolution of this equation shows that there are two qualitatively very different forms of behavior depending on the sign of the chromatic dispersion (D):
First case: D&gt;0. In this case, phenomena of instability of modulation are observed. The pulses "burst" into very short pulses at the end of 1000 to 2000 km and the optical spectrum widens considerably: this may give rise to problems related to the optical passband. PA1 Second case: D&lt;0. There is no instability of modulation and the pulses keep a certain degree of integrity while the spectrum widens quasi-monotonically during the propagation, while keeping reasonable widths. However, the pulses widen greatly temporally, thus creating inter-symbol interferences. These interferences become very troublesome for example, as soon as the chromatic dispersion goes beyond 0.05 ps/nm/km in terms of absolute value for bit rates of 5 Gbits/s over distances of 6000 to 8000 km. PA1 means for the amplitude modulation of said digital signal, delivering an amplitude modulated signal; PA1 means for the modification of the instantaneous optical frequency of said amplitude modulated signal, at the instants corresponding to said rising and descending edges of said digital signal. PA1 the optical power along said transmission line; PA1 the length of said optical fiber; PA1 the coefficient of chromatic dispersion of said optical fiber; PA1 the bit rate of said digital signal; PA1 the binary coding format of said digital signal; PA1 the distance between two repetition amplifiers placed on said transmission line; PA1 the noise excess factor of said repetition amplifiers. PA1 A(t) is the amplitude of said optical field, PA1 .alpha. is a constant, PA1 t represents time, PA1 an electro-optical amplitude modulator, controlled by an electrical signal (D) representing said digital signal and delivering an amplitude modulation signal, and PA1 a phase modulator, acting on said amplitude modulated signal and controlled by said electrical signal (D) delayed by a period that is substantially equal to the delay introduced by said amplitude modulator.
The most promising case is naturally the second one, that of a negative chromatic dispersion. However, to make the very-long-distance systems work in negative dispersion, the values of chromatic dispersion of the fibers used must obligatorily be very low.
2. Description of the Prior Art
Any method of compensation for the two phenomena that are the cause of the distortion in the fiber (chromatic dispersion and non-linear effects) is therefore very useful since it can be used to overcome the drawback of the low values imposed on the chromatic dispersion. Indeed, by using a method of compensation for the distortion provided by the transmission fiber, it is possible to envisage two strategies.
In a first strategy, for given characteristics of negative chromatic dispersion of the transmission fiber, the compensation can be used to increase the product: line bit rate * range of the link.
In a second strategy, for a fixed line bit rate and a fixed range, the compensation makes it possible to use transmission line fibers having the least stringent constraints as regards the characteristics of chromatic dispersion. These fibers are easier to manufacture on an industrial scale and to sort out for the setting up of an underwater link for example.
There are known methods of compensation at the source of transmission for distortions induced by the optical fiber line.
Thus one method, described for example by L. Koch and R. Alferness, in "Dispersion Compensation by Active Predistorted Signal Synthesis" in the Journal of Lightwave Technology (vol. LT3, No. 4, August 1985) consists in making the pulses undergo a continuous scanning of the optical frequency, for the duration of one bit, by over-modulation of the transmission laser.
For example, a pulse designed to be transmitted through a negative dispersion fiber (which will push the high frequency components towards the front edge of the pulse) will undergo an over-modulation of optical frequency in addition to the electro-optical amplitude modulation so that the high-frequency components are towards the rear edge of the pulse.
What is used here is the fact that the optical frequency of a semiconductor laser depends on its current. However, only certain structures of lasers can be applied to this method, for they display great "frequency modulation efficiency", i.e. a major variation of this optical frequency for a small variation in the control current and hence in the optical power.
Furthermore, this known method of compensation at the source of transmission is suited solely to compensation for the penalties entailed by chromatic dispersion. Furthermore, it considers only disturbances of this type. It does not directly relate to compensation for non-linear effects.
This method also has the drawback of necessitating a particular over-modulation that is synchronous with the useful digital train, on the transmission laser used. Consequently, only certain types of lasers, capable of giving a major variation of the optical frequency for a small variation in the control current, can be used in this method.
Finally, this method is applicable only to an on-line RZ code.